The present disclosure relates generally to battery chargers for electric vehicles and more particularly to chargers that must perform with a range of supply voltages.
A typical circuit for charging the large traction battery in an EV from the AC supply may be similar to that illustrated in FIG. 1, with input filtering omitted. Charging circuit 100 includes a six diode full wave bridge rectifier 150 (with no connection to neutral) that feeds a single DC to DC converter 180. The DC to DC converter 180 converts the voltage from the rectifier 150 to that needed to charge the traction battery. The voltage needed to charge the battery varies over a significant range as the battery charges from 0% state of charge to 100% state of charge. This range may span a ratio of 1.6 to 1 for example.
Charger 100 of FIG. 1 may be sufficient when no variation in input voltage from the grid is expected, such as in Europe, where three phase 230/400 VAC (Wye/Delta) is commonly available and high power single phase is not common. In some European jurisdictions, high power single phase electricity is not allowed while it is common in the United States.
Electric Vehicle (EV) battery charging from high power three phase utility voltages in the Americas encounters a range of supply voltages and input topologies. In the United States for example, the supply voltages can be 480/277 volts (Delta/Wye) three phase, 208/120 volts three phase or 240 volts “split phase” (single phase with center tap to neutral).
The typical EV battery charger 100 of FIG. 1 offers rated power at the highest supported input voltage with a linear decrease as the input voltage decreases. A 11 kW charger at 480/230 volts input might provide less than 5 kW at 208/120 input for example. This is due, in part, to the fact that it must deal with AC input voltage variation in addition to the DC output voltage variation as the battery charges.
The standard three-phase charging connector (IEC 62196 and SAE J3068) is often used in passenger car applications with only single-phase wiring. In this case, power is supplied between the pins for line 1 and neutral. If such a connection is supplied to the charger of FIG. 1, no power flows since this charger does not make use of the neutral connection. This incompatibility is illustrated in FIG. 2 which shows only one connection to the three-phase charger when a typical single-phase supply connection is used. This inflexibility is a serious limitation to the charger, as such single-phase connections through a three-phase connector are common.
In order to handle such variation of the input voltage while maintaining reasonable output power, current ratings of all the components used in the charging circuit can be increased. However, higher current rating increases the cost of the components.
Another approach may include changing the circuit from one that implements an inexpensive boost converter topology to one that implements a more expensive buck-boost topology. Input voltage variation are less of a problem at low power such as that used in laptop computer power supplies which must operate at different voltages in different countries. It becomes prohibitively expensive as power ratings exceed 10 kW.
Alternatively, three independent isolated battery chargers can be used to improve performance over a wide input voltage range. This single phase charger trio is connected either in a Delta topology for low-input-voltage ranges or in a Wye topology for high input voltages.
In a known system, such as that illustrated in FIG. 3, a charging device 300 includes three independent, isolated chargers 315, 325 and 335 (which are illustrated as AC/DC converters) with one input of each converter connected to phases 310, 320 and 330 respectively of a three phase grid supply. The other input of converters 315, 325 and 335 are connected to switches 318, 328 and 338 respectively. The switches can either be in position A or in position B, depending on the grid supply voltage.
The grid supply transformer secondary is configured in Wye topology. The assumption is that the converter inputs can accommodate an input voltage range between 208 VAC and 277 VAC nominal. The switches can also be implemented as jumpers with different settings for different market regions if individual products are not expected to encounter varying supply voltages thus enabling one device to serve various markets with a simple jumper change.
If the supply voltage is 208/120 VAC, each of switches 318, 328 and 338 should be set to the corresponding “A” position for best efficiency. In this setting, each of the converters is connected in delta (or A) topology to the three phase supply. Converter 315 is connected between grid supply phases 310 and 330, converter 325 is connected between grid supply phases 320 and 310, and converter 335 is connected between grid supply phases 330 and 320. Each of the converters 315, 325 and 335 therefore, sees an input voltage of 208 VAC.
If the supply voltage is 480/277 VAC, switches 318, 328 and 338 have to be set to the corresponding “B” position (as position “A” would present an over-voltage input condition). In this setting, each of the converters is connected between one phase of the three phase grid supply and to a neutral line 340. This is known as Wye (or Star) topology. Each converter 315, 325 and 335 sees an input voltage of 277 VAC.
The voltage range from 208 VAC to 480 VAC represents a ratio of 2.3 (480÷208 or 277÷120). This indicates that a charging circuit such as FIG. 1 has to handle a variation range of 130% if it must work with both 208/120 and 480/277 VAC supplies.
Using the circuit of FIG. 3, the voltage that needs to be handled ranges from 208 VAC to 277 VAC which is a ratio of 1.3 (277÷208). The converters 315, 325 and 335, therefore, see an input voltage increase of 30% when changing from 208/120 to 480/277 grid supplies with the benefit of the prior art in FIG. 3, versus a voltage increase of 130% without this topology change as seen in FIG. 1.
A single phase 240 VAC supply can also be used with the three charger system of FIG. 3, yielding good performance. As illustrated in FIG. 4, charging device 400 is connected to a single phase 240 VAC supply 410 via an adapter 405. Switches 418, 428 and 438 corresponding to converters 415, 425 and 435 are set to the “B” position. Converters 415, 425 and 435 each see an input voltage of 240 VAC (Position “A” is not used with the single phase adapter 405, as position A would present zero volts to the converter inputs).
Power at 240 VAC single-phase input is potentially about 87% (240÷277) of the 480/277 VAC three-phase case. Even if a single phase input adapter only connects to one of the three input phases (as might be the case if “Electric Vehicle Supply Equipment” for single phase passenger EV is connected), charging device 400 can still supply potentially one third of maximum rated power. This overcomes the limitations of the charging circuit 200 of FIG. 2.
While heavy commercial EVs primarily need the higher power afforded by three phase charging connections such as SAE J3068, the ability to take advantage of the significant installed base of single phase public SAE J1772 charging points adds significant value in many use cases. As of the end of 2014, over twenty thousand public J1772 single charging points are available, while there are as yet no J3068 three phase charging points as the J3068 standard is still under development.
The use of the conventional three chargers, however, is potentially costly and a more economical solution is desirable, especially if it retains flexibility in accommodating wide input voltage ranges and accommodating various single phase input topologies.